Our experience w/ the kids areas is that you must sign them in and out. Our son has been on four cruises so far and this has been the process each time. He is now ten and will be going on his fifth next month. Not sure about the age (around 10-12), but if you give permission, they will allow the child to leave on his/her own.
We brought our own walkie/talkies w/ us. He will call when he was ready to leave.
You should contact your TA and have them check if they rent pagers.